Hans von Euler-Chelpin was the son of a Bavarian soldier and originally intended to become an artist. He studied at the Munich Academy of Painting where Franz von Lenbach was his teacher. Because he wanted to find out what is behind the colors, however, he discontinued his artistic education and turned to science. He studied chemistry, first in Munich, then in Berlin, where he received his doctorate in 1895. Two years later, he moved to Stockholm where he was appointed a lecturer (Privatdozent) in physical chemistry, fell in love with the Swedish chemist Astrid Cleve and became a Swedish citizen when he married her in 1902. The very curiosity and energy that drove him to get acquainted to the chemical side of colors characterized his whole career as a scientist, which culminated in receiving a Nobel Prize in Chemistry in 1929 together with Arthur Harden „for their investigations on the fermentation of sugar and fermentative enzymes". The repercussion of this energy and the wealth of his knowledge can be felt in this lecture that he delivered in Lindau at the age of 85. In von Euler-Chelpin’s family there surely is a genetic imprinting for scientific excellence, with the eminent mathematician Leonhard Euler being one of his ancestors, and his son Ulf a co-recipient of the Nobel Prize in Physiology or Medicine in 1970.„The relation between the chemical constitution and the effect of an active ingredient is an old problem of biochemistry and chemistry, especially with regard to pharmaceutical drugs“, von Euler-Chelpin introduces this talk, his third one at the Lindau Meetings, which he attended seven times between 1951 and 1960. „Numerous rules have been determined according to which the chemical structure of a drug can successfully be modified. By systematic alteration based on empirical findings the efficacy of a compound can be increased to a certain maximum.“ Yet von Euler’s intent lies not in informing his audience about the progress of medicinal chemistry or research subjects of pharmaceutical interest, he primarily discusses „some remarkable cases, in which structural influences have an impact on the biochemistry of the cell“, with an emphasis on carcinogenic processes. He eludes, for example, on polycyclic aromatic hydrocarbons whose carcinogenicity can be increased by the introduction of methyl groups. He mentions compounds, in which aromatics are linked via amino groups: Methyl yellow („Buttergelb“) is the most toxic of these compounds, but looses all its carcinogenicity when stripped off its two methyl groups. He reflects on the role of n-oxidation and seeks for a molecular explanation why the first cytotoxic chemotherapeutic agents for the treatment of cancer (nitrogen mustards) were structurally derived from mustard gas („N-Lost“). He shares his thoughts on the consequences of the n-oxidation of nucleic acids and speculates about the involvement of nicotinamide adenine dinucleotide (NAD+), the coenzyme whose functional elucidation had earned him a Nobel Prize.The 1950s were the early years of chemotherapeutic treatment of cancer and many current insights still lay far ahead. Yet with two important textbooks, published in 1942 and 1962, respectively, von Euler contributed substantially to promote and teach this field. Without doubt, he met the expectations of the Swedish Academy „that the distinction which has fallen upon you today, will not contain for you the temptation to rest on laurels already obtained, but that on the contrary it will mean a stimulus to continued and, as we all hope, successful work in the service of biochemistry“. [1]Joachim Pietzsch[1] cf. Award Ceremony Speech: http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1929/press.html